1. Field of the Invention
The present invention relates to refrigerator cabinets which are classified in Class 312, subclass 314. More particularly, the present invention relates to composite structures for use as thermal insulation panels used in the fabrication of cabinets for refrigerators, freezers, ice boxes, picnic coolers, and the like (hereinafter referred to as refrigerators).
2. Description of the Prior Art
Thermal insulation panels of the type used in refrigerators usually comprise a thermally insulating cellular material laminated to an impact resistant thermoplastic sheet which forms the interior surface of the food compartment of the refrigerator. The interior surface of the food compartment, including the inside of the door, which is presented to the food stored in the refrigerator, is hereinafter referred to as the refrigerator liner.
In most refrigerator construction, gas filled polyurethane foam is used as the cellular insulation material and acrylonitrile-butadiene-styrene (ABS) polymer or rubber modified polystyrene is used as the refrigerator liner. The gas found in the cells of the insulation material is usually a halogenated aliphatic or cycloaliphatic hydrocarbon. These gases are present in the cells of the insulation material as residual blowing agents. Alternately, they are incorporated into the cells by design in order to improve the thermal insulation properties of the cellular material. Examples of these hydrocarbons include trichlorofluoromethane, dichlorodifluoromethane, 1,1,2-trichloro-1,2,2-trifluoroethane, and mixtures thereof.
The composites described above have found widespread use in refrigerator applications. Unfortunately, these composites suffer from a disadvantage in that the food liner becomes less impact resistant and more susceptible to cracking when bonded to the gas filled foam. The loss of impact resistance and the tendency to crack is reduced by the use of an elastomeric layer between the gas filled foam and the food liner as taught in U.S. Pat. No. 3,563,845 to J. Stevens. In U.S. Pat. No. 3,565,746, the same patentee teaches the use of two different layers of gas filled cellular material, which layers differ in density and compressive modulus in order to minimize loss of impact strength. Other methods used in the art to preserve the impact strength of the ABS layer include the use of a wax layer or a layer of polyethylene between the cellular material and the ABS.
The methods described above provide some improvement in the retention of impact resistance of the ABS refrigerator liners upon being laminated to the gas filled cellular material.
However, the impact resistance of the ABS refrigerator liners may still deteriorate due to the aging of the elastomeric or wax layers which are interposed between the cellular material and the ABS food liner. In addition, the halogenated hydrocarbons in the cellular material may permeate the elastomeric, wax and polyethylene layers and attack the ABS sheet surface. In either event the result is a decrease in impact strength of the ABS sheet in the composite structure.
While the use of wax or polyethylene interlayers between the foam and ABS sheet will eliminate stresses and strains which are formed in the ABS by virtue of being laminated to the foam, they give rise to another problem. The wax and polyethylene layers provide total release of the ABS from the foam and as a result the ABS food liner of the refrigerator tends to lose rigidity and may flex back and forth upon contact. This loss of rigidity may be avoided by the use of an adhesive layer wherein the wax coating or the polyethylene is bonded to both the urethane foam and the ABS sheet, however, this, in turn, gives rise to additional manufacturing steps and expenses.
Another disadvantage of the methods used in the prior art is that the elastomeric, wax or polyethylene components must be separated from the ABS material in order to recycle any ABS trimmings, scraps, etc. generated in the manufacturing operations back into food liner use as it has been found that the physical properties of the ABS food liner component become less desirable when they are contaminated with minor amounts of the above material. This gives rise to further manufacturing steps if the trimmings are to be recycled for food liner applications.
A need exists in the art for improved thermal insulation panels for refrigeration applications wherein the ABS refrigerator liner is protected against the halogenated hydrocarbons found in the cellular insulation material used in refrigerator application while maintaining rigidity and impact strength.